The present embodiments relate to wireless communications systems and are more particularly directed to an orthogonal frequency division multiplexing (“OFDM”) system.
Wireless communications are now prevalent in many applications, including both business and personal communication systems. The present embodiments have particular application in such systems and particularly those that are sometimes referred to as fixed wireless systems. Fixed wireless systems are so named because the last distance of the downliik communication, typically on the order of one or two miles, is expected to include a wireless communication to a device that is not mobile or, if mobile, has a very slow fading characteristic. For example, a fixed wireless system in contemporary applications may include wireless communications to a modem inside a home or business.
One wireless technique that has had favorable use in a fixed environment is OFDM, which also is introduced here as it has particular application to the preferred embodiments described later. By way of introduction to OFDM, the more general frequency division multiplexing (“FDM”) is characterized by transmission of multiple signals simultaneously over a single transmission path, such as a wireless system. Each of the multiple signals travels at a different frequency band, sometimes referred to as a carrier or sub-carrier, and which is modulated by the data. More particularly, each sub-carrier is actually a sinc (sin(x)/x) function. In any event, the data carried by each sub-carrier may be user data of many forms, including text, voice, video, and the like. In addition, the data includes control data, a particular type of which is discussed below. In any event, OFDM was developed several years ago, and it adds an element of orthogonality to FDM. In OFDM, the center frequency of each of the sub-carriers are spaced apart at specific frequencies, where the frequency spacing is such that each sub-carrier is orthogonal to the other sub-carriers. As a result of the orthogonality, ideally each receiving element tuned to a given sub-carrier does not perceive any of the signals communicated at any other of the sub-carriers. Given this aspect, various benefits arise. For example, OFDM is able to use overlapping (while orthogonal) sub-carriers and, as a result, thorough use is made of the overall OFDM spectrum. As another example, in many wireless systems, the same transmitted signal arrives at the receiver at different times, that is, having traveled different lengths due to reflections in the channel between the transmitter and receiver; each different arrival of the same originally-transmitted signal is typically referred to as a multipath. Typically multipaths interfere with one another, which is sometimes referred to as intersymbol interference (“ISI”) because each path includes transmitted data referred to as symbols. Nonetheless, the orthogonality implemented by OFDM considerably reduces ISI and, as a result, often a less complex receiver structure, such as one without an equalizer, may be implemented in an OFDM system. Lastly, note that OFDM also has been used in mobile wireless communications, and is currently being developed in various respects including in combination with other wireless communication techniques.
While OFDM communications have proven useful and indeed beneficial in various contexts, the present inventors have recognized certain drawbacks in OFDM. For example, in present OFDM applications, data are transmitted in a form that is sometimes referred to as an OFDM symbol, which is a collection of parallel data assigned to different sub-carriers and communicated as a group. Within this OFDM symbol some of the sub-carriers carry data that is not user data but instead that is control data that describes to the receiver information about the coding and modulation scheme then being used by the transmitter. These control data are sometimes referred to as training tones or training control data and part of the information they carry is sometimes referred to as a code parameter set (“CPS”) or generally as transmission parameter signaling. In present OFDM systems, however, the present inventors have observed that the CPS information provided by the training tones is relatively stagnant. Particularly, a typical OFDM system repeatedly communicates the same CPS information in every Nth successive OFDM symbols, where N equals three. For example, often a transmitter will maintain the same CPS information for all operating time between successive resets. Then, at each reset event, each receiver must be put in some neutral state while the transmitter begins to transmit a new set of CPS information with each Nth successively-transmitted OFDM symbol. Thereafter, each receiver then operates according to the new CPS information. Further, because all receivers operate according to the same CPS information, then typically the transmitter selects the CPS information so as to accommodate the weakest communication channel existing among all of the receivers. As a result, subsequent communications to all receivers are based on this worst-case-established CPS and, thus, performance with respect to the receivers that could benefit from different CPS information are instead constrained by the performance of the weakest channel. Also as a result of the above-described manner of communicating CPS information, there is limited flexibility in what may be described by the CPS information. In contrast, the preferred embodiments seek to increase the scope of flexibility provided by CPS and other control information, which brings still other benefits, all of which are discussed in greater detail below.